Image carriers for projection and especially for overhead projection systems are usually clear plastic sheets. The information to be marked or projected is in the form of written or drawn representations applied to the carrier sheet as black or colored images formed by writing or drawing on the carrier sheet.
The overhead projector is normally used under daylight projection conditions, often not in darkened rooms. Therefore, the markings on the sheet must be sharply contrasting as well as legible. Until now, generally black or colored markings to form the images were provided on a white background. However, recently on testing, it was found that legibility could be greatly improved or enhanced when a color contrast was added to the brightness contrast. When the light and dark (black/white) portions of the image or illustration do not have the same color and/or are other than black and white, the distinctiveness and/or recognition factors are enhanced.
Tests have shown that the best legibility and recognition is attained when a bright yellow image or writing appears on a contrasting dark blue background. Until recently such bright images on dark backgrounds could only be satisfactorily produced by using opaque colors or dyes. This, as is well known in the art, is not practical in projections systems where transparency of the field and image colors are required. When opaque colors are used the opaque areas are projected as black areas.
It has been proposed in DOS No. 27 51 122 to produce images on dark-toned backgrounds or fields by means of pH-indicator dyes. If an indicator dye is used, which shifts from blue to yellow based upon the pH applied to the dye (alkaline-blue; acidic-yellow), there may be prepared an overall blue layer of the indicator dye under alkaline conditions which is coated on the carrier sheet and on which image markings may be made by means of an acidic marking device. Such markers are the usual fiber-tipped writing materials which are charged with a liquid acidic medium. Where such an acid has marked the carrier, a bright yellow image is formed on the overall blue field.
Such acid-basic indicator systems, however, have the decisive drawback that they react to trace acids such as are present in the perspiration, particularly from the hands, as well as trace acids present in the polluted atmosphere such as from acidic mists generated in and near power plants where fossil fuels are burned.
Such acid mists are very common in urban environments. Further, the handling of the carriers during projection or during the making of the images, will often cause bright yellow fingerprints to appear on the blue fields in areas of inadvertent contact of the hand with the carrier.
It is also well known that the pH-sensitive dyes are characterized by poor light resistance. By the frequent re-use of the carrier sheets in the projection system, these dyes tend to fade and the overall intensity and contrast between the marked and unmarked areas is reduced. Thus, such pH systems have the drawback of being sensitive to handling, sensitive to the environment, and subject to fading upon extended exposure to light or repeated use.
It is an object of the present invention to provide image carriers for projection systems which overcome the above drawback.
It is another object of the present invention to provide overhead projection sheets on which may be produced brightly colored images or writings on contrastingly colored backgrounds, particularly yellow image writings on blue backgrounds.
It is a further object of the present invention to provide carrier sheets on which the images are light-resistant and insensitive to contamination during handling or from the polluted acidic environment of urban areas.
It is a further object of this invention to provide a carrier sheet having good image contrast and a method for providing such high contrast images on such projection carrier sheets which is based on acid-insensitive dyes.